Operations and Significant Accounting Policies - Supplemental Statement of Cash Flows Information - USD ($) $ in Millions	6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019
Supplemental Cash Flow Information [Abstract]
Cash Paid for Interest – Net of Amounts Capitalized	$ 30.9	$ 33.8
Recognition of Right-of-use Assets and Lease Liabilities	0.0	31.2
Noncash Investing and Financing Activities [Abstract]
Increase (Decrease) in Accounts Payable for Capital Additions to Property, Plant and Equipment	(67.5)	21.1
Capitalized Asset Retirement Costs	2.1	1.4
AFUDC–Equity	$ 1.4	$ 1.3